1. Field of the Invention
This invention relates to an improved process for preparing the through hole walls of a printed wiring board (PWB) for electroplating.
2. Description of Related Art
For the past quarter century the printed wiring board industry has relied on the electroless copper deposition process to prepare through hole walls in printed wiring boards for electroplating. These plated through hole walls are necessary to achieve connections between two metal circuit patterns on each side of a printed wiring board or, in addition to this, between the inner layer circuit patterns of a multilayer board.
The electroless deposition of copper onto the through hole walls typically consists of precleaning a PWB and then processing according to the following sequence of steps:
Step 1. Preactivator PA1 Step 2. Pd/Sn Activator PA1 Step 3. Rinse PA1 Step 4. Acellerator PA1 Step 5. Rinse PA1 Step 6. Electroless Copper Deposition PA1 Step 7. Electroplating PA1 (a) contacting said printed wiring board having said through holes with a liquid dispersion of carbon black comprised of: PA1 (b) separating substantially all of the liquid dispersing medium from said applied dispersion, thereby depositing said carbon black particles in a substantially continuous layer on said non-conducting portions of said hole walls; PA1 (c) microetching said metal layers of said printed wiring board to remove deposited carbon black therefrom; and PA1 (d) electroplating a substantially continuous metal layer over the deposited carbon black layer on said non-conducting portions of hole walls, thereby electrically connecting said metal layers of said printed wiring board; PA1 contacting said deposited carbon black particles after step (b) and before step (c) with aqueous solution of an alkali metal borate.
These processed boards may also be photoimaged before the electroplating process. Typically, the deposited copper layer on each through hole wall is about 1+0.2 mil thick.
Conventional electroless processes have several commercial disadvantages. They require a relatively long process time. The multiple treatment baths have complex chemistry which may require constant monitoring and individual ingredients which may require separate replenishment. The palladium/tin activator also may require expensive waste treatment. Furthermore, these electroless process baths may be very sensitive to contamination. Finally, the multiplicity of rinse baths may require large amounts of water.
Prior to the electroless method of plating through holes, graphite was employed to prepare the walls of the through holes for plating. For example, U.S. Pat. No. 3,099,608, which issued to Radovsky et al. on July 30, 1963, teaches a process for preparing the through hole walls of printed circuit boards for electroplating by initially depositing in said through holes a thin electrically non-conductive film of palladium metal in at least a semi-colloidal form. The patent discloses that graphite had been used previously as a conductive layer for electroplating thereon. See column 1, lines 63-70 and column 4, line 72 to column 5, line 11. These patentees noted several deficiencies with that graphite process including lack of control of the graphite application, poor deposit of the resultant electroplated metal, non-uniform through hole diameters, and low electrical resistance of the graphite.
U.S. Pat. No. 3,163,588, which issued to Shortt et al on Dec. 29, 1964, also mentions that graphite or its equivalents may be employed to render through hole walls of electric circuit boards conductive for later electroplating metals thereon. See column 3, line 45 to column 4, line 2.
U.S. Pat. No. 4,581,301, which issued to Michaelson on Apr. 8, 1986, teaches the application of a seed layer of conductive particles, such as "carbon", on the wall of through holes before electrolytically plating copper over the seed layer. This reference does not explicitly teach the use of a continuous layer of surfactant or carbon black in the seed layer, and does not recognize the advantage of using very small particles of carbon black such as presently claimed. See column 7, lines 63-66 which refer to particles passing through a 400 mesh screen. A 400 mesh screen is equivalent to about 35 microns.
Separately, graphite has been employed in numerous processes for preparing a non-conducting material for a metal coating or plating. For example, U.S. Pat. No. 409,096, which issued to Alois Blank on Aug. 13, 1889, teaches a process for applying copper to asbestos roofing material which comprises first applying powdered plumbago (graphite) in a volatile liquid such as varnish to the surface of the asbestos, then evaporating the volatile liquid to coat the asbestos fibers with fine particles of plumbago. The plumbago coated asbestos sheets are then immersed in a copper electroplating solution and electric current is applied to the coated asbestos sheet to form a thin film of copper thereon. The copper coated sheet is then immersed in a bath of molten metal such as tin, lead, or zinc, and is then removed from the molten bath to effect solidification of the molten metal. The resulting metal coated asbestos sheet is described as being relatively flexible, a non-conductor of heat and substantially fireproof.
U.S. Patent No. 1,037,469, which issued to Goldberg on Sept. 3, 1912, and U.S. Pat. No. 1,352,331, which issued to Unno on Sept. 7, 1920, disclose processes for electroplating non-conducting materials by first coating the non-conducting material with wax, then coating the wax with a slurry of finely divided particles of graphite or other metal, followed by electroplating of the dust-coated surface with copper or other metal. Neither of these processes are particularly suitable for use in coating the hole walls of circuit boards because the holes are normally extremely narrow in diameter and immersing in wax would tend to plug the hole and prevent coating the hole walls with an electroplating material.
U.S. Pat. No. 2,243,429, which issued to Laux on May 27, 1941, discloses a process for electroplating a non-conductive surface by "graphiting" a thin layer onto the non-conducting surface followed by applying a copper layer electrolytically and "finally a further electrolytic deposit of another metal" is placed thereon.
Separately, caroon black formulations have been employed as conductive coatings for non-conductive materials. For example, U.S. Pat. No. 4,035,265, which issued to Saunders on July 12, 1977, discloses conductive paint compositions containing both graphite and carbon black along with air-hardenable binder. These paints are suitable for application to the walls of a building for use as a heating element.
U.S. Pat. No. 4,090,984, which issued to Lin et al on May 23, 1978, teaches a semi-conductive coating for glass fibers comprising (a) a polyacrylate emulsion; (b) electrically conductive carbon black dispersion and (c) a thixotropic gelling agent. The conductive carbon black dispersions employed are those comprising electrically conductive carbon black dispersed in from about 3 to about 4 percent by weight of a suitable dispersing agent.
U.S. Pat. No. 4,239,794, which issued to Allard on Dec. 16, 1980, teaches dispersing a conductive carbon black in a latex binder with a selected dispersing agent, then impregnating this carbon black dispersion into a non-woven fibrous web followed by drying any residual water, leaving a thin coating of carbon black dispersed on the surfaces of said fibers.
U. S. Pat. Nos. 4,619,741; 4,684,560 and 4,724,005, which issued to Karl L. Minten and Galina Pismennaya, on Oct. 28, 1986; Aug. 4, 1987; and Feb.9, 1988, respectively, teach a process of electroplating the through holes of a PWB which is a significant improvement over the known electroless techniques. By this process, a liquid dispersion of carbon black particles is first applied to the non-conductive portions of the through holes; then the liquid dispersion medium is separated (i.e. evaporated) from the carbon black particles, thereby depositing a substantially continuous layer of carbon black particles on the non-conductive surfaces of the through holes; and next a substantially continuous metal layer is electroplated over the deposited carbon black layer. This process of Minten and Pismennaya has several advantages over the known electroless techniques including the eliminaton of the preactivator, the Pd/Sn activator and the accelerator; less possibility of pollution problems; better bath stability; and fewer possible side reactions. This disclosure of the above-mentioned U.S. Patents of Minten and Pismennaya is incorporated herein by reference in their entirety.
While this Minten and Pismennaya process in itself teaches an effective means for electroplating through holes, there is still a need to improve the overall quality (i.e. achieve a void-free copper deposit) for all types of printed wiring boards, especially multilayer boards. Furthermore, while a preferred embodiment disclosed in this application employs a microetch to remove carbon black particles from the copper on metal surfaces of the PWB before electroplating, there is still a need to more thoroughly remove the carbon black off of these metal surfaces. In other words, it desirable to make that microetching step more effective.
This is especially necessary for copper foil rims of the though holes of the PWB and the inner copper foil layers inside the through holes of the PWB to be completely cleaned before the micro-etch step. If there is a relatively thick carbon black layer remaining on those rims or inner copper foil layers during the electroplating step, that carbon black might insulate those rims or copper foil layers from the plated-on copper layer and, thus prevent a good electrolyic connection between them, and, if the carbon black layer is plated over in these rim and inner layer areas, an adhesion failure may occur. Also, in some instances, there is loose excess deposits of carbon black on the non-conductive (e.g. epoxy/fiberglass) portions of the through holes. If electroplating proceeds with these loose excess deposits present, the plating surface over these portions may not be smooth.
Improvement and modifications of this Minten and Pismennaya process are shown in U.S. Pat. Nos. 4,622,107 (Piano); 4,622,108 (Polakovic and Piano) and 4,631,117 (Minten, Battisti and Pismennaya) and 4,718,993 (Cupta and Piano). The first of these patents teaches the use of a gas-forming compound (e.g. sodium carbonate) to remove loose or easily removable carbon black particles in the through holes. The second of these patents teaches the contacting of an alkaline hydroxide pre-conditioning solution to the through hole walls before application of the carbon black dispersion so that the carbon black dispersion will have better adhesion to the walls. The third listed patent teaches the use of this carbon black dispersion as a pre-activator for electroless plating of the through holes. The fourth teaches the use of a alkaline silicate solution before the carbon black dispersion. These four U.S. Pats. are incorporated herein by reference in their entireties.
It is a primary object of this invention is to provide improved electroplating process for applying a conductive metal layer to the through hole walls of printed wiring boards over the process disclosed in the above-noted Minten and Pismennaya patents.
It is another object of this invention to provide an even more economical process for applying a conductive metal layer to the surfaces of non-conducting layers of printed wiring boards than presently known electroless processes.